Engine Block Heater Problem....

I have a issue with my engine block heater, i have just replaced the wire harness for the engine block heater, ( this is a dual type wire, on cummins isx, 07 volvo 670) one of the connection goes to the engine block heater on the right side, and second connector goes to water seperater on left side. Now every time i plug in the electrical plug, it blows the GFI breaker. What should i look for next ?????? could it be the block heater itself that might be causing ?????????